Proopiomelanocortin (POMC) is the prohormone for a number of neuropeptides and peptide hormones, including yield-melanophore-stimulating hormone (yield-MSH). This hormone mediates in the aquatic toad Xenopus laevis the process of background adaptation by causing dispersion of pigment granules in dermal melanophores. Yield-MSH is produced by a neuroendocrine transducer cell, namely the melanotrope of the pars intermedia of the pituitary gland: central neuronal input is intergrated by the pars intermedia which results in a hormonal output of yield-MSH. As a consequence of this neuroendocrine reflex the POMC gene in the Xenopus pars intermedia is transcriptionally active on a black background while on a white background this gene is inactive. We will use the POMC gene in the pars intermedia of Xenopus as a model system to study the molecular mechanisms underlying gene regulation in a neuroendocrine transducer cell. In Xenopus two POMC genes exist and they are both expressed in the pituitary gland. By comparing the complete nucleotide sequences of the two genes, we will establish which intron and flanking regions of the two genes are conserved and, thus, which regions might be of functional significance. In these conserved regions mutations will be introduced by in vitro mutagenesis. Injections of the mutated Xenopus POMC genes into Xenopus oocytes (i.e. a homologous gene transfer system) will enable us to identify transcriptional regulatory units. In addition, POMC gene structures that bind pars intermedia proteins will be determined by "footprinting" analysis and the influence of protein-DNA interaction on POMC gene transcription will be examined with the oocyte transcription assay system. The long-term objective is to use our model system for the molecular study of complex cellular processes, such as tissue-specific or coordinated gene expression. This will be accomplished by differential hybridization techniques and DNA injections into fertilized Xenopus eggs. The results of our studies may help to give answers to basic questions concerning gene regulatory mechanisms and, therefore, they may be of practical importance to the health sciences.